Yes it's working now, indeed I did exactly what you suggested just before you made your kind suggestion. ANyway, thanks very much for your help!
Regarding the other question that I encountered 404 error after upgrade, this is actually a problem encountered by many users. There is an extremely sticky long post (with more than 200 replies) in the phpBB forum regarding this, and this problem apparently is due to a bug in the problem. People have suggested amendments to some of the files, which are quite troublesome in my opinion. So my suggestion to those who want to have phpBB 2.0.4: manually install the whole thing through FTP instead of installing 2.0.2 through Cpanel and then upgrade.
Why 2.0.4? As the phpBB website says, bug fixes and important security improvement